The radio transmitter is an important part of, for example mobile telephony systems. A transmitter in radio communications equipment will comprise, among other things, a power amplifier in which transistors are included. A transistor will generate heat at high powers. Typical problems that may occur are overheating and parameter changes. Overheating can damage the transistor and render it totally unserviceable. A change in temperature can change the parameters of the transistor and result in interferences. To prevent overheating, and therewith transistor damage, or parameter changes and therewith interferences, it is important that the transistor junction temperature can be measured correctly. When the temperature of a transistor is known, damage can be prevented or compensation made for any parameter changes that may occur. This is important and often essential in systems in which high performance is required. Mobile telephone systems are examples of such systems.
One typical method of measuring the temperature of a power transistor is to place a temperature sensor in the proximity of the heat-conducting outer casing of the transistor, for instance on a cooling fin. This method is unsatisfactory in many cases, since it is then the temperature of the outer casing that is measured rather than the junction temperature of the transistor. The measured values are therewith thermally delayed as a result of the physical distance between the active transistor parts and the temperature sensor. The temperature measurement is also misleading, since the temperature value decreases with the distance between the transistor and the temperature sensor. There is a danger that a sudden increase in temperature will not be measured until it is too late, in other words until the transistor has already been damaged. To prevent this, it is necessary to introduce large safety margins, which, in turn, increases costs. Other methods of measuring the temperature of a transistor are known to the art. U.S. Pat. No. 5,383,083 describes how the temperature of a transistor is measured by connecting a measuring circuit to the transistor base and registering the voltage across the base-emitter of the transistor, and converting the measurement to a temperature value. One drawback with this technique is that normal transistor operation will disturb the temperature measuring process, or vice versa that the measuring process will disturb the normal operation of the transistor.